This invention relates to photonics, and in particular to a method for selectively shifting the bandgap in quantum well microstructures.
The rapid growth of fiber-optic based applications for communications has resulted in the proliferation of manufacturing sites capable of keeping up with the growing demand of photonic devices. The fiber-optic component market is expect to increase from $6.7 billion in 1999 to over $23 billion in 2003. In order to keep with this growth, the current manufacturing methods that depend heavily on labor extensive assembly of individual devices, have to give a way to technologies capable of manufacturing integrated devices, in a manner similar to what has been taking place since 1970s in the microelectronics industry. Generally, current manufacturing technologies of photonics devices deal separately with active (lasers, amplifiers, switches) and passive (waveguides, filters, microlenses) components. This is mostly due to the incompatibility of technologies that are required for manufacturing both passive and active device materials.
Advances in fiber optics communication technology have resulted in the deployment of systems, carrying different wavelengths of light on a single fiber. The wavelength division multiplexing (WDW) technology is elegant, scalable and most effective in delivering increased bandwidth. The most crucial device of WDM is a quantum well (QW) semiconductor laser. The currently used lasers are distributed feedback (DFB) type discrete devices with thermoelectric coolers (TEC) for fine wavelength tuning and maintaining stable wavelength within different operating conditions.
There is a demand for duplicate wavelengths in communication systems with high reliability where spare channels have to be provisioned to provide backup in a case of transmitter failure. This need can be fulfilled with tunable lasers or multi wavelength lasers.
Wavelength tunability can be achieved by constructing a Bragg grating within the laser structure. The tunable lasers are usually DFB lasers with multiple segments of distributed Bragg reflectors integrated within semiconductor structure (e.g., C. Clarke et al., xe2x80x98Tunable Lasers Provide Flexible Optical Routing Solutions, Laser Focus World, April 1999, p. 77).
Another approach is to provide fiber Bragg grating in an external cavity DFB, laser structure. Tuning in this case involves mechanical movement of the grating and thus is relatively slow.
An alternative solution is a hybrid array of multi-wavelength lasers built within the same package and usually equipped with an optical combiner for signal distribution. Such a hybrid consists of lasers that are selected from different wafers; based on the wavelength they emit, and usually don""t exceed 16 lasers in one package.
The single laser sources for WDM systems require a bulky package. Individually enclosed lasers, usually in butterfly package, occupy significant space within system subassemblies.
Individual fiber pigtails demand attention during assembly and testing, and require management at subsystem levels. Discrete laser wavelengths necessitate maintaining numerous (and costly) module inventory as spare circuits/modules by network operators.
Both tunable and multi-wavelength lasers exhibit a number of limitations. These devices have a limited tuning range (typically less than 20 nm) and they are relatively expensive considering technological difficulties in large-scale fabrication.
Tunable sources are slow when switching from one wavelength to another and this may be a serious drawback for certain network architectures.
The cost of DFB lasers modules and tunable sources is still prohibitive in penetrating data networking segments such as LANs, MANs and future residential high-bandwidth access market. The cost of an individual uncooled laser and a high performance cooled laser for communication applications runs at about U$200 and U$ 1,400, respectively. A 4-wavelength laser hybrid costs in excess of U$18,000.
The paper xe2x80x9cSemiconductor Laser Array Fabricated by Nd:YAG Laser-induced Quantum Well Intermixingxe2x80x9d, by J. J. Dubowski, G. Marshall, Y. Feng, P. J. Poole, C. Lacelle, J. E. Haysom, S. Charbonneau and M. Buchanan, SPIE vol. 3618, 191-197, describes an idea of creating a material with continuously changing bandgap that would be useful for the fabrication of multiwavelength semiconductor laser arrays. The method is based on the application of an IR laser with a beam profile shaped in such a way that a gradient of temperature would be induced on an irradiated semiconductor wafer. The principle of this method is based entirely on temperature-induced quantum well intermixing.
While this proposal offers a partial solution to the problems outlined above, it is based entirely on temperature-induced quantum well intermixing. Such approach is limited in its ability to shift the band gap with a sufficient lateral resolution (contrast) required for manufacturing of micrometer-scale integrated photonic devices. An object of the invention is to provide an improved solution to the problems occurring in the prior art.
According to the present invention, there is provided a method of selectively shifting the bandgap of a quantum well microstructure, comprising providing a microstructure containing quantum wells, irradiating a surface of said microstructure with changing ultra violet (UV) radiation to cause alteration of a near-surface region, said ultra violet radiation being varied across said surface to change the amount of said alteration, said alteration being selected from the group consisting of defect formation and chemical alteration, and subsequently annealing the microstructure to induce quantum well intermixing mediated by said alteration and thereby cause a bandgap shift dependent on said ultra violet radiation.
The microstructure may be selectively irradiated in a predetermined pattern, for example, in the form of an array, or alternatively the whole microstructure can be irradiated while changing the ultraviolet radiation, for example, radiation dose, intensity or wavelength, across the surface of the microstructure in order to provide a continually changing bandgap across the wafer in the finished device.
The invention makes use of UV photons to induce defect formation or cause chemical alteration of the near-surface region. The use of UV photons for defect formation (or changing chemical composition of the material at the surface) is very efficient since this is a direct process: no ion implantation, or impurity doping are required. Therefore, it is a much faster and potentially a much more cost-effective process than alternative methodologies.
The use of UV lasers for bandgap shifting is, in its simplest approach, a two-step process. First, the laser is used to produce defects and/or chemically altered material at the surface of a quantum well microstructure. Since the penetration of the UV radiation into the sample is strongly dependent on the laser wavelength, a precise amount of defects (composition of the surface layer) can be achieved in the near-surface region by choosing a proper laser and the irradiation dose. The UV irradiation is a highly selective laterally process, with processed areas comparable to the laser spot size (sub-micrometer resolution).
In the second step, the laser processed wafer (selectivity is used to generate required patterns) is annealed for a very short time (10-15 sec) and defect-enhanced quantum well intermixing is achieved. Annealing at his stage can be carried out in a conventional RTA (Rapid Thermal Anneal) furnace, or with an IR laser (to maintain high selectivity of the process and allow for in-situ diagnostics). Large amplitudes of the bandgap shifting (more than 100 nm) are achieved with this method.
The technology is based on the application of UV lasers for formation of surface defects and/or an xe2x80x9caltered layerxe2x80x9d of material at the surface of processed sample (wafer). The concentration and physical nature of the defects, as well as the chemical composition of the altered layer affect the quantum well intermixing (QWI) process that takes place during annealing of the UV processed material. The lateral resolution of the process makes it possible to fabricate micro-arrays of monolithically integrated multiwavelength lasers. For instance, for a 3 xcexcm wide ridge structure, a laser chip consisting of an array of 100-200 lasers each laser emitting at a fixed but different wavelength, could be fabricated on a 2-3 mm long bar. The same technology can be used to fabricate waveguides integrated with each laser from the array or more complex integrated photonic microstructures.
The invention permits the fine tuning of the optical properties of wafers used for manufacturing of semiconductor lasers. The lateral selectivity of the process makes possible tuning of individual lasers from multi-wavelength laser arrays.
The invention can be applied to manufacturing technology for processing epitaxial wafers of photonic materials and fabrication of inexpensive monolithically integrated devices, such as arrays of multi-wavelength lasers, waveguides, laser-waveguide microstructures and other optical components. The same technology can also be used for semiconductor laser facet treatment, which should lead to better performance devices.
The predominant application of devices fabricated with the use of this invention will likely be in local area networks (LAN), metropolitan area networks (MAN) where fast tuning of the transmitter is essential to the system performance. The number of wavelengths in those applications doesn""t have to be as large as in dense WDM long haul systems, and thus the requirement for channel spacing can be also relaxed. Also, in wavelength routed networks those laser arrays will be advantageous, considering cost and wavelength selection feature.
The invention is particularly useful in the fabrication of optical components in a monolithically integrated geometry that would involve lasers, waveguides, beam splitters, optical switches and gratings required for processing of optical signals.